Polyolefin coated steel pipes

ABSTRACT

Polyolefin coated steel pipes with high dynamic fracture toughness of the coating of the steel pipes during installation handling and in service, consisting of a steel pipe core, optionally an intermediate foamed plastic material, and a polyolefin coating of β-nucleated propylene copolymers whereby a test polyolefin pipe fabricated from the β-nucleated propylene copolymer has a critical pressure of &gt;25 bars and a dynamic fracture toughness &gt;3.5 MNm −3/2 . The polyolefin coated steel pipes are suitable for off-shore transport of crude oil or gas products or district heating applications.

FIELD OF THE INVENTION

The invention relates to polyolefin coated steel pipes with high dynamicfracture toughness of the coating of the steel pipes during installationhandling and in service consisting of a steel pipe core, optionally anintermediate foamed, filled or solid plastic material, and a polyolefincoating, as well as a process for producing them.

BACKGROUND OF THE INVENTION

Polyolefin coated steel pipes with a polyolefin coating consisting oflinear low density polyethylene (JP 08,300,561), blends of propylenepolymers and α-olefin copolymer elastomers (JP 2000,44,909) orsyndiotactic polypropylene (JP 08,300,562) are known. The disadvantageof these polyolefin steel coatings is the insufficient dynamic fracturetoughness of test pipes fabricated from the coating material. A highdynamic fracture toughness is required for coated steel pipes in orderto avoid cracking of the coating during installation handling and inservice.

The term installation handling as used herein means any installationtechnique such as coiling and uncoiling of the ready made pipelines,welding and other jointing techniques and installation at the seabottomfor off-shore intallations with specially designed ships, most often toa depth of several hundreds of meters, also to uncertain sea bottomconditions with risk of rock impingements etc. Installation handling ofcoated steel pipes, in particular for off-shore applications, involvestough conditions for the protective coating layer, including highstress, substantial elongation, surface damages, notches, impact eventsetc, both at low and high temperature conditions and also at highhydrostatic pressure. The coating layer is not only the layer protectingthe pipeline as such from damages as mentioned, it is also doing so in astage of high stress and/or at elevated temperatures and pressures,making the coating layer most sensitive for cracking, compare inparticular the stresses induced during coiling and uncoiling. During theservice life of the coated pipeline, the coating has to protect thepipeline from damages and induced stress and crack formations atconditions close to 0° C., high hydrostatic pressures where a smalldamage or notch in the coating could propagate into a large crackputting the pipeline as such at risk. With a high dynamic fracturetoughness of the coating material the material will not crack duringinstallation handling and in service.

OBJECT OF THE INVENTION

It is the object of the present invention to provide polyolefin coatedsteel pipes with high dynamic fracture toughness of the coating of thesteel pipes during installation handling and in service.

BRIEF DESCRIPTION OF THE INVENTION

According to the present invention, this object is achieved bypolyolefin coated steel pipes with dynamic fracture toughness of thecoating of the steel pipes during installation handling and in service,consisting of a steel pipe core, optionally an intermediate foamedplastic material, and a polyolefin coating, wherein the polyolefincoating consists of β-nucleated propylene copolymers from 90.0 to 99.9wt % of propylene and 0.1 to 10.0 wt % of α-olefins with 2 or 4 to 18carbon atoms with melt indices of 0.1 to 8 g/10 min at 230° C./2.16 kg,whereby a test polyolefin pipe fabricated from the β-nucleated propylenecopolymer has a critical pressure of >25 bars and a dynamic fracturetoughness of >3.5 MNm^(−3/2) in the hydrostatic small scale steady state(hydrostatic S₄) test at 3° C.

DETAILED DESCRIPTION OF THE INVENTION

The term installation handling as used herein means any installationtechnique such as coiling, uncoiling, welding and other jointingtechniques.

β-nucleated propylene polymers are isotactic propylene polymers composedof chains in a 3₁ helical conformation having an internal microstructureof β-form spherulites being composed of radial arrays of parallelstacked lamellae. This microstructure can be realized by the addition ofβ-nucleating agents to the melt and subsequent crystallization. Thepresence of the β-form can be detected through the use of wide angleX-ray diffraction (Moore, J., Polypropylene Hand-book, p.134-135, HanserPublishers Munich 1996).

According to an advantageous embodiment, the β-nucleated propylenecopolymers of the polyolefin coating are β-nucleated propylene blockcopolymers having an IRτ ≧0.97. More preferably, the β-nucleatedpropylene block copolymers have an IRτ ≧0.98, a tensile modulus of≧11100 MPa at +23° C. and a Charpy impact strength, notched, ≧6 kJ/m² at−20° C. It is even more preferable for the β-nucleated propylene blockcopolymers to have an IRτ of ≧0.985. The difference of 0.005 in IRτ, IRτbeing a measure for isotacticity, encompasses a significant increase inmechanical polymer properties, especially in stiffness.

The IRτ of the propylene polymers is measured and calculated asdescribed in EP 0 277 514 A2 on page 5 (column 7, line 53 to column 8,line 11).

The propylene copolymers for use as coating for steel pipes according tothe present invention have melt indices of 0.1 to 8 g/10 min at 230°C./2.16 kg, preferably 0.2 to 5 g/10 min at 230° C./2.16 kg.

According to a further preferred embodiment the β-nucleatedpolypropylene block copolymers have a tensile modulus of preferably≧1300 MPa and most preferably ≧1500 MPa at +23° C.

Charpy impact strength of the β-nucleated propylene copolymers is ≧6kJ/m² at −20° C., preferably ≧9 kJ/m² at −20° C., most preferably ≧10kJ/m² at −20° C. Charpy impact strength of up to at least 60 kJ/m² ispossible for copolymers.

Dynamic fracture toughness calculated from the critical pressure in thehydrostatic small scale steady state (S₄) test of test pressure pipes isan important safety parameter for steel pipe polyolefin coatingmaterials with high dynamic fracture toughness of the polyolefin coatingof the coating of the steel pipes during installation handling and inservice.

The method of determining the dynamic fracture toughness is disclosed inPlastics, Rubber and Composites Processing and Applications, Vol. 26,No.9, pp.387 ff.

The dynamic fracture toughness K_(D) is calculated directly from thehydrostatic S4 test critical pressure P_(c) at 3° C. according tofollowing equation:K _(D) =p _(c)(πD/7)^(1/2)·(D*2),wherein P_(c) is the critical pressure, D is the diameter of the testpipe and D* is D/t and t is the wall thickness of the test pipe.

Comparative values for critical pressure [bar] and dynamic fracturetoughness [MNm^(−3/2)] for common steel coating materials areapproximately 7.44/bar1.5 MNm^(−3/2) for propylene-ethylene randomcopolymer. These materials are not suitable as polyolefin coatingmaterials with high crack toughness of the coating of the steel pipesduring coiling, uncoiling, installation handling and in service. For thepropylene-ethylene random copolymers dynamic fracture toughness isinsufficient for the proposed applications in steel pipe coatings.

According to a further embodiment, the β-nucleated propylene blockcopolymers of the polyolefin coating having an IRτ of the propylenehomopolymer block of ≧0.98 are propylene copolymers obtained bypolymerization with a Ziegler-Natta catalyst system comprisingtitanium-containing solid components, an organoalumina, magnesium ortitanium compound as cocatalyst and an external donor according to theformulaR_(x)R′_(y)Si(MeO)_(4-x-y),wherein R and R′ are identical or different and are branched or cyclicaliphatic or aromatic hydrocarbon residues, and y and x independentlyfrom each other are 0 or 1, provided that x+y are 1 or 2.

A preferred external donor in the Ziegler-Natta catalyst system forproducing the β-nucleated propylene block copolymers of the polyolefincoating of the steel pipes is dicyclopentyldimethoxysilane.

According to an advantageous embodiment the β-nucleated propylenecopolymers of the polyolefin coating contain 0,0001 to 2,0 wt %, basedon the propylene copolymers used,

-   -   dicarboxylic acid derivative type diamide compounds from        C₅-C₈-cycloalkyl monoamines or C₆-C₁₂-aromatic monoamines and        C₅-C₈-aliphatic, C₅-C₈-cycloaliphatic or C₆-C₁₂-aromatic        dicarboxylic acids, and/or    -   diamine derivative type diamide compounds from C₅-C₈-cycloalkyl        monocarboxylic acids or C₆-C₁₂-aromatic monocarboxylic acids and        C₅-C₈-cycloaliphatic or C₆-C₁₂-aromatic diamines, and/or    -   amino acid derivative type diamide compounds from amidation        reaction of C₅-C₈-alkyl-, C₅-C₈-cycloalkyl- or C₆-C₁₂-arylamino        acids, C₅-C₈-alkyl-, C₅-C₈-cycloalkyl- or C₆-C₁₂-aromatic        monocarboxylic acid chlorides and C₅-C₈-alkyl-,        C₅-C₈-cycloalkyl- or C₆-C₁₂-aromatic mono-amines, and/or    -   quinacridone derivative compounds of the type quinacridone        compounds, quinacridonequinone compounds, and/or        dihydroquinacridone type compounds, and/or    -   dicarboxylic acid salts of metals from group IIa of periodic        system and/or mixtures of dicarboxylic acids and metals from        group IIa of periodic system, and/or    -   salts of metals from group IIa of periodic system and imido        acids of the formula        wherein x=1 to 4; R═H, —COOH, C₁-C₁₂-alkyl, C₅-C₈-cycloalkyl or        C₆-C₁₂-aryl, and Y═C₁-C₁₂-alkyl, C₅-C₆-cycloalkyl or        C₆-C₁₂-aryl-substituted bivalent C₆-C₁₂-aromatic residues,    -   as β-nucleating agent.

Examples of the dicarboxylic acid derivative type diamide compounds fromC₅-C₈-cycloalkyl monoamines or C₆-C₁₂-aromatic monoamines andC₅-C₈-aliphatic, C₅-C₈-cycloaliphatic or C₆-C₁₂-aromatic dicarboxylicacids, optionally contained in the β-nucleated propylene copolymers ofthe polyolefin coating of the steel pipe, are

-   -   N,N′-di-C₅-C₈-cycloalkyl-2,6-naphthalene dicarboxamide compounds        such as        -   N,N′-dicyclohexyl-2,6-naphthalene dicarboxamide and        -   N,N′-dicyclooctyl-2,6-naphthalene dicarboxamide,    -   N,N′-di-C₅-C₈-cycloalkyl-4,4-biphenyidicarboxamide compounds        such as        -   N,N′-dicyclohexyl-4,4-biphenyidicarboxamide and        -   N,N′-dicyclopentyl-4,4-biphenyidicarboxamide,    -   N,N′-di-C₅-C₈-cycloalkyl-terephthalamide compounds such as        -   N,N′-dicyclohexylterephthalamide and        -   N,N′-dicyclopentylterephthalamide,    -   N,N′-di-C₅-C₈-cycloalkyl-1,4-cyclohexanedicarboxamide compounds        such as        -   N,N′-dicyclohexyl-1,4-cyclohexanedicarboxamide and        -   N,N′-dicyclohexyl-1,4-cyclopentanedicarboxamide.

Examples of the diamine derivative type diamide compounds fromC₅-C₈-cycloalkyl monocarboxylic acids or C₆-C₁₂-aromatic monocarboxylicacids and C₅-C₈-cycloaliphatic or C₆-C₁₂-aromatic diamines, optionallycontained in the β-nucleated propylene copolymers of the polyolefincoating of the steel pipe, are

-   -   N,N′-C₆-C₁₂-arylene-bis-benzamide compounds such as        -   N,N′-p-phenylene-bis-benzamide and        -   N,N′-1,5-naphthalene-bis-benzamide,    -   N,N′-C₅-C₈-cycloalkyl-bis-benzamide compounds such as        -   N,N′-1,4-cyclopentane-bis-benzamide and        -   N,N′-1,4-cyclohexane-bis-benzamide.    -   N,N′-p-C₆-C₁₂-arylene-bis-C₅-C₈-cycloalkylcarboxamide compounds        such as        -   N,N′-1,5-naphthalene-bis-cyclohexanecarboxamide and        -   N,N′-1,4-phenylene-bis-cyclohexanecarboxamide.    -   N,N′-C₅-C₈-cycloalkyl-bis-cyclohexanecarboxamide compounds such        as        -   N,N′-1,4-cyclopentane-bis-cyclohexanecarboxamide and        -   N,N′-1,4-cyclohexane-bis-cyclohexanecarboxamide.

Examples of the aminoacid derivative type diamide compounds, optionallycontained in the β-nucleated propylene copolymers of the polyolefincoating of the steel pipe, are N-phenyl-5-(N-benzoylamino)-pentaneamideand/or N-cyclohexyl-4-(N-cyclohexylcarbonylamino)-benzamide.

Examples of the quinacridone type compounds, optionally contained in theβ-nucleated propylene copolymers of the polyolefin coating of the steelpipe, are quinacridone, dimethylquinacridone and/ordimethoxyquinacridone.

Examples of the quinacridonequinone type compounds, optionally containedin the 1-nucleated propylene copolymers of the polyolefin coating of thesteel pipe, are quinacridonequinone, a mixed crystal of5,12-dihydro(2,3b)acridine-7,14-dione withquino(2,3b)acridine-6,7,13,14-(5H,12H)-tetrone as disclosed in EP-B 0177 961 and/or dimethoxyquinacridonequinone.

Examples of the dihydroquinacridone type compounds, optionally containedin the β-nucleated propylene copolymers of the polyolefin coating of thesteel pipe, are dihydroquinacridone, dimethoxydihydroquinacridone and/ordibenzodihydroquinacridone.

Examples of the dicarboxylic acid salts of metals from group IIa ofperiodic system, optionally contained in the β-nucleated propylenecopolymers of the polyolefin coating of the steel pipe, are pimelic acidcalcium salt and/or suberic acid calcium salt.

Examples of salts of metals from group IIa of periodic system and imidoacids of the formula

optionally contained in the β-nucleated propylene copolymers of thepolyolefin coating of the steel pipe, are the calcium salts ofphthaloylglycine, hexahydrophthaloylglycine, N-phthaloylalanine and/orN4-methylphthaloylglycine.

According to an advantageous feature of the present invention, theintermediate foamed plastic material, being optionally interposedbetween the steel pipe and the polyolefin coating, is a foamed propylenecopolymer having strain hardening behaviour and a melt index of 1.5 to10 g/10 min at 230° C./2.16 kg.

The propylene copolymer of the intermediate foamed plastic material,being optionally interposed between the steel pipe and the polyolefincoating, having a strain hardening behaviour can be produced by anynumber of processes, e.g. by treatment of propylene copolymers withthermal decomposing radical-forming agents and/or by treatment withionizing radiation, where both treatments may optionally be accompaniedor followed by a treatment with bi- or multifunctionally unsaturatedmonomers, e.g. butadiene, isoprene, dimethylbutadiene or divinylbenzene.Further processes may be suitable for the production of the propylenecopolymers having a strain hardening behaviour, provided that theresulting propylene copolymers meet the characteristics of strainhardening behaviour.

Examples of said propylene copolymers of the intermediate foamed plasticmaterial, being optionally interposed between the steel pipe and thepolyolefin coating, having a strain hardening behaviour are, inparticular:

-   -   polypropylenes modified by the reaction of polypropylenes with        bismaleimido-compounds in the melt (EP-A-0 574 801; EP-A-0 574        804),    -   polypropylenes modified by the treatment of polypropylenes with        ionizing radiation in the solid phase (EP-A-0 190 889; EP-A-0        634 454),    -   polypropylenes modified by the treatment of polypropylenes with        peroxides in the solid phase (EP-A-0-384 431) or in the melt        (EP-A-0-142724),    -   polypropylenes modified by the treatment of polypropylenes with        multifunctional, ethylenically unsaturated monomers under the        action of ionizing radiation (EP-A-0 678 527)    -   polypropylenes modified by the treatment of polypropylenes with        multifunctional, ethylenically unsaturated monomers in the        presence of peroxides in the melt (EP-A-0 688 817; EP-A-0 450        342)

The strain hardening behaviour as used herein is defined according toFIGS. 1 and 2.

FIG. 1 shows a schematic representation of the experimental procedurewhich is used to determine strain hardening.

The strain hardening behaviour of polymers is analyzed by Rheotensapparatus 1 (product of Göttfert, Siemensstr. 2, 74711 Buchen, Germany)in which a melt strand 2 is elongated by drawing down with a definedacceleration. The haul-off force F in dependence of drawn down velocityv is recorded.

The Rheotens apparatus 1 is combined with an extruder/melt pump 3 forcontinuous feeding of the melt strand 2. The extrusion temperature is200° C.; a capillary die with a diameter of 2 mm and a length of 6 mm isused and the acceleration of the melt strand 2 drawn down is 120mm/sec².

The schematic diagram in FIG. 1 shows in an exemplary fashion themeasured increase in haul-off force F (i.e. “melt strength”) vs. theincrease in draw-down velocity v (i.e. “drawability”).

FIG. 2 shows the recorded curves of Rheotens measurements of polymersamples with and without strain hardening behaviour. The maximum points(F_(max); v_(max)) at failure of the strand are characteristic for thestrength and the drawability of the melt.

The common propylene polymers 4, 5, 6 with melt indices of 0.3, 2.0 and3.0 g/10 min at 230° C./2.16 kg show a very low melt strength and lowdrawability. They have no strain hardening and therefore a problematicprocessability into extrusion foams. Modified propylene polymers 7 (meltindex of the sample in the diagram is 2 to 3 g/10 min at 230° C./2.16kg) or unmodified common LDPE 8 (melt index of the sample in the diagramis 0.7 g/10 min at 190° C./2.16 kg) show a completely differentdrawability vs. melt strength behaviour. With increasing the draw downvelocity v the haul-off force F increases to a much higher level,compared to the unmodified common propylene polymers 4, 5, 6. The curveshape is characteristic for strain hardening.

“Propylene copolymers which have strain hardening behaviour” as usedherein have enhanced melt strength with haul-off forces F_(max)>15 cNand enhanced drawability velocities v_(max)>150 mm/s.

According to a further preferred embodiment of the invention the testpolyolefin pipe fabricated from the β-nucleated propylene copolymer hasa critical pressure of >30 bars and a dynamic fracture toughness of >6.0MNm^(−3/2) in the hydrostatic small scale steady state (hydrostatic S₄)test at 3° C.

A further object of the present invention is a process for producingpolyolefin coated steel pipes with high dynamic fracture toughness ofthe coating of the steel pipes during installation handling and inservice, consisting of a steel pipe core, optionally an intermediatefoamed plastic material, and a polyolefin coating fabricated by coatingextruder/rotating steel pipe technology, ring-die pipe coatingtechnology or injection molding technology, characterized in that thepolyolefin coating consists of β-nucleated propylene copolymers from90.0 to 99.9% by weight of propylene and 0.1 to 10.0% by weight ofα-olefins with 2 or 4 to 18 carbon atoms with melt indices of 0.1 to 8g/10 min at 230° C./2.16 kg, whereby a test polyolefin pipe fabricatedfrom the β-nucleated propylene copolymer has a critical pressure of >25bars and a dynamic fracture toughness of >3.5 MNm^(−3/2) in thehydrostatic small scale steady state (hydrostatic S4) test at 3° C.

The inventive propylene block copolymers for the coating of steel pipesmay contain usual auxiliary materials, such as 0.01 to 2.5 wt %stabilizers and/or 0.01 to 1 wt % processing aids, and/or 0.1 to 1 wt %antistatic agents and/or 0.2 to 3 wt % pigments, in each case based onthe propylene copolymers used.

As stabilizers preferably mixtures of 0.01 to 0.6 wt % phenolicantioxidants, 0.01 to 0.6 wt % 3-arylbenzofuranones, 0.01 to 0.6 wt %processing stabilizers based on phosphites, 0.01 to 0.6 wt % hightemperature stabilizers based on disulfides and thioethers and/or 0.01to 0.8 wt % sterically hindered amines (HALS) are suitable.

For a good interlaminar adhesion between the steel pipe core, optionallythe intermediate foamed plastic material, or the polyolefin coating itis advantageous to use epoxy resin coated steel pipes and to apply acompatibilizing layer between the epoxy resin coated steel pipe and thepolyolefin layer, whereby the compatibilizing layer consists ofpropylene copolymers or propylene polymer graft copolymers both withchemical bound ethylenically unsaturated carbonic acids and/or carbonicacid anhydrides, particularly acrylic acid, methacrylic acid and/ormaleic acid anhydride.

Conventional extruders for melting the propylene copolymers pursuant tothe inventive process.

Producing the polyolefin coated steel pipes by coating extruder/rotatingsteel pipe technology, the preheated steel pipe, optionally coated withan epoxy resin, under rotation successively is melt coated byindependent coating extruders having flat film dies for the unfoamedpolyolefin cover layer and the optional layers of the compatibilizingagent and the foaming plastic material.

Producing the polyolefin coated steel pipes by crosshead die pipecoating technology, preferably a crosshead fed by extruders, for theouter unfoamed polyolefin cover layer and optional for thecompatibilizing agent and the foaming plastic material, is used. Thesteel pipe is preferably coated with an epoxy resin layer and acompatibilizing layer on the epoxy resin layer. Preferably the steelpipe is preheated to a temperature ranging from 170 to 230° C., and theextruder feeding the ring shaped die of the crosshead in the polyolefinsteel pipe coating line has a temperature profile ranging from 175 to250° C. The optional foamed melt is brought first on the pipe, followedby the unfoamed outer layer of the β-nucleated propylene copolymer,subsequently the coated pipe is calibrated in the calibrating sleeve andcooled. Preferred are steel pipe diameters ranging from 50 to 500 mm.

Injection molding technology for producing the polyolefin coated steelpipes is used at field joint. The field joint coating machine consistsof two parts. The injection molding machine melts the β-nucleatedpropylene copolymer in an extruder with adapter zones and then injectsit into the mold, which is controlled by the mold locking part. In thissecond part the β-nucleated propylene copolymer is cooled down to solidstate by oil or water. The preferred temperature profile of the extruderis from 200 to 250° C. and of the adapter zones from 230 to 240° C. Thepreferred mold temperature is from 80 to 100° C.

If optional an intermediate foamed plastic material is applied on thesteel pipe, preferred polyolefin mixtures containing 1 to 12 wt %, basedon the polyolefin mixture, of chemical blowing agents that split offgas, or hydrocarbons, halogenated hydrocarbons and/or gases as blowingagents are used, whereby the steel pipes are preheated to a temperatureranging from 170 to 230° C. and the foam coating extruder has atemperature profile ranging from 175 to 250° C.

Examples of suitable chemical blowing agents, that emit a gas, aresodium hydrogencarbonate, azodicarbonamide and/or cyanuric trihydrazide.Suitable hydrocarbons as blowing agents are readily volatilehydrocarbons, such as pentane, isopentane, propane and/or isobutane.Examples of suitable halogenated hydrocarbons aremonofluorotrichloromethane and/or difluoromonochloromethane. Suitablegases as blowing agents are nitrogen, argon and/or carbon dioxide.

According to a feature of the present invention in the ring-die pipecoating technology for producing the polyolefin coated steel pipes acone extruder is used, whereby the temperature of the melt of thenucleated propylene copolymer at the ring die is from 195 to 240° C. andthe temperature of the preheated steel pipe is from 160 to 200° C.

Preferred applications of polyolefin coated steel pipes are theoff-shore transport of crude oil or gas products or district heatingapplications.

In the application as polyolefin coated steel pipes for off-shoretransport of crude oil from sea bottom to tankers, coated steel pipeswith an intermediate foamed propylene copolymer material with foamdensities of the foamed layer ranging from 600 to 800 kg/m³ arepreferred. In order to be able to pump crude oil coming from deposit incold sea regions, the fluid has to be held sufficiently warm. Byutilising the inventive coated steel pipes with an intermediate foamedpropylene copolymer based insulation layer, it is possible to avoidextensive heat losses to the surrounding water and also to eliminatecostly additional oil heating units along the pipe line. At water depthsof 200 to 300 m the pressure is substantial and requires high mechanicalstability of the foamed insulation layer. The foam layers of propylenecopolymers having strain hardening behaviour have the outstandingbalance between heat insulation efficiency and compression strength.

EXAMPLES

The following tests were made using injection molded test specimenprepared according to ISO 1873

Tensile modulus according to ISO 527 (cross head speed 1 mm/min) at +23°C.

Charpy impact strength, notched according to ISO 179/1eA

Rapid crack propagation test according to ISO 13477 performed underhydrostatic conditions

Dynamic fracture toughness according to Plastics, Rubber and CompositesProcessing and Applications, Vol. 26, No. 9, pp.387 ff.

Compressive strength according to ASTM D 695-96, 5% compression

Example 1

1.1 Preparation of the β-Nucleated Propylene Copolymer

A mixture of 90 wt % of a propylene block copolymer, obtained bycombined bulk and gas phase polymerization using a Ziegler-Nattacatalyst system with dicyclopentyldimethoxysilane as external donor,having an ethylene content of 8.3 wt %, an IRτ of the propylenehomopolymer block of 0.985 and a melt index of 0.30 g/10 min at 230°C./2.16 kg, 10 wt % of a master batch comprising 99 parts by weight of apropylene block copolymer having an ethylene content of 8.3% by weight,an IRτ of the propylene homopolymer block of 0.985 and a melt index of0.30 g/10 min at 230° C./2.16 kg, and 1 part by weight of pimelic acidcalcium salt, and 0.1 wt % calcium stearate, 0.1 wt %tetrakis[methylene(3,5-di-t-butylhydroxyhydrocinnamate)]methane and 0.1wt % tris-(2,4-di-t-butylphenyl)phosphite, based on the sum of thepropylene polymers used, is melted in a twin screw extruder with atemperature profile of 100/145/185/210/220/225/225/225/220/200/185° C.,homogenized, discharged and pelletized.

The resulting propylene copolymer has a melt index of 0.32 g/10 min at230° C./2.16 kg, a tensile modulus of 1290 MPa and a Charpy impactstrength, using notched test specimens, of 39 kJ/m² at −20° C.

1.2 Manufacture of the Propylene Copolymer Test Pipe

For producing the propylene copolymer test pipe, the β-nucleatedpropylene copolymer of 1.1 is introduced in a single screw extruder(L/D=30, D=70 mm, temperature profile 200/210/220/220/220/220/200°0 C.,40 rpm), melted, extruded through a ring shaped die with a diameter of110 mm, taken off over a vacuum calibrating sleeve as a pipe of adiameter of 110 mm and a wall thickness of 10 mm, and cooled in a 6 mwater bath at +20° C., the taking off velocity being 0.3 m/min.

Rapid crack propagation test shows a critical pressure of 31 bar and adynamic fracture toughness of 19.60 MNm^(−3/2).

1.3 Manufacture of the Polyolefin Coated Steel Pipe

The pilot steel pipe coating line consists of a preheating unit,crosshead with two extruders, vacuum calibration sleeve, cooling unitand cutting unit.

For producing the intermediate foamed plastic layer, a propylene polymercompound comprising

-   -   30 wt % of a propylene homopolymer modified with 0.12% by weight        of bound butadiene as determined by IR-spectroscopy and having        strain hardening behaviour, a melt index of 0.45 g/10 min at        230° C./2.16 kg and a crystallization enthalpy of 91 J/g,    -   70 wt % of a propylene block copolymer having an ethylene        content of 8.3% by weight, an IRτ of the propylene homopolymer        block of 0.974, and a melt index of 0.30 g/10 min at 230°        C./2.16 kg, and 0.1 wt % calcium stearate, 0.1 wt %        tetrakis[methylene(3,5-di-t-butylhydroxyhydrocinnamate)]methane        and 0.1 wt % tris-(2,4-di-t-butylphenyl)phosphite, based on the        sum of the propylene polymers used, is dry blended with 2.2 wt        %, based on the propylene compound, of a mixture of blowing        agents based on bicarbonate and citric acid and supplied by        means of a metering system to the feeding funnel of the first        single screw extruder with a screw diameter of 90 mm, an L/D of        35 and a temperature profile of        200/230/240/230/230/23012301230/230/230° C. Initially, the        mixture is melted and homogenized and subsequently the split off        blowing gas is mixed intensively in the extruder and distributed        homogeneously. After that, the melt is transferred by a melt        pump onto the ring-shaped crosshead having a die temperature of        205° C.

The said crosshead is fed by a second single screw extruder with a screwdiameter of 60 mm, a L/D of 35 and a temperature profile of200/230/240/220/220/220/220/220/220/220° C. with the β-nucleatedpropylene copolymer of 1.1.

Inside of the crosshead a steel pipe (Ø150 mm), coated with a 25 μmepoxy resin layer and a 30 μm compatibilizing layer of a maleic acidanhydride grafted propylene polymer (0.20 wt % maleic acid anhydride),being preheated to a temperature of 190° C., is driven forward with aspeed of 1.2 m/min. The crosshead is designed so that the foamed melt isadded first onto the coated steel pipe, followed by the melt of theunfoamed propylene polymer for the outer layer, just before the pipeenters the vacuum calibration sleeve, which is cooled by water of +20°C.

From the polyolefin coated steel pipe test specimens of a length of 254mm are machine cut. The polyolefin foam layer has a thickness of 50 mmand a density of 720 kg/m³. The unfoamed cover layer has a thickness of8 mm. The compressive strength of coated steel pipe test specimens (ASTMD 695-96, 5% compression) being 19 MPa.

Example 2

2.1 Preparation of the β-Nucleated Propylene Copolymer

A mixture of 94 wt % of a propylene block copolymer, obtained bycombined bulk and gas phase polymerization using a Ziegler-Nattacatalyst system with dicyclopentyldimethoxysilane as external donor,having an ethylene content of 8.3 wt %, an IRτ of the propylenehomopolymer block of 0.985, and a melt index of 0.30 g/10 min at 230°C./2.16 kg, 6 wt % of a master batch comprising 99.8 parts by weight ofa propylene block copolymer having an ethylene content of 8.3 wt %, anIRτ of the propylene homopolymer block of 0.985, and a melt index of0.30 g/10 min at 230° C./2.16 kg, and 0.2 parts by weight of a mixedcrystal of 5,12-dihydro(2,3b)acridine-7,14-dione withquino(2,3b)acridine-6,7,13,14-(5H,12H)-tetrone, and 0.05 wt % calciumstearate, 0.1wt %tetrakis[methylene(3,5-di-t-butylhydroxyhydrocinnamate)]methane and0.1wt % tris-(2,4-di-t-butyl-phenyl)-phosphite, based on the sum of thepropylene polymers used, is melted in a twin screw extruder with atemperature profile of 100/145/190/215/225/230/230/215/205/190° C.,homogenized, discharged and pelletized.

The resulting polypropylene polymer has a melt index of 0.3 g/10 min at230° C./2.16 kg, a tensile modulus of 1450 MPa and a Charpy impactstrength using notched test specimens at −20° C. of 21 kJ/m².

2.2 Manufacture of the Propylene Copolymer Test Pipe

For producing the propylene copolymer test pipe, the β-nucleatedpropylene copolymer of 2.1 is introduced in a single screw extruder(L/D=30, D=70 mm, temperature profile 200/210/225/225/225/225/205° C.,40 rpm), melted, extruded through a ring shaped die with a diameter of110 mm, taken off over a vacuum calibrating sleeve as a pipe of adiameter of 110 mm and a wall thickness of 10 mm, and cooled in a 6 mwater bath at 20° C., the taking off velocity being 0.35 m/min.

Rapid crack propagation test shows a critical pressure of 34 bar and adynamic fracture toughness of 21.5 MNm^(−3/2).

2.3 Manufacture of the Polyolefin Coated Steel Pipe

The pilot steel pipe coating line consists of a preheating unit,crosshead with two extruders, vacuum calibration sleeve, cooling unitand cutting unit.

For producing the intermediate foamed plastic layer, a propylene polymercompound comprising

-   -   20 wt % of a polypropylene copolymer having an ethylene content        of 4.3 wt %, modified with 0.16% by weight of bound        divinylbenzene, as determined by IR-spectroscopy, and having        strain hardening behaviour and a melt index of 0.48 g/10 min at        230° C./2.16 kg,    -   80 wt % of a propylene block copolymer having an ethylene        content of 8.3 wt %, an IRτ of 0.974 of the propylene block, and        a melt index of 0.30 g/10 min at 230° C./2.16 kg, and 0.1 wt %        calcium stearate, 0.1 wt %        tetrakis[methylene(3,5-di-t-butylhydroxyhydrocinnamate)]methane        and 0.1 wt % tris-(2,4-di-t-butylphenyl)phosphite, based on the        sum of the propylene polymers used, is dry blended with 2.2 wt        %, based on the propylene compound, of a mixture of blowing        agents based on bicarbonate and citric acid and supplied by        means of a metering system to the feeding funnel of the first        single screw extruder with a screw diameter of 90 mm, an L/D of        35 and a temperature profile of        200/230/240/230/230/230/230/230/230/230° C.

Initially, the mixture is melted and homogenized and subsequently thesplit off blowing gas is mixed intensively in the extruder anddistributed homogeneously. After that, the melt is transferred by a meltpump onto the ring-shaped crosshead having a die temperature of 205° C.

The said crosshead is fed by a second single screw extruder with a screwdiameter of 60 mm, an L/D of 35 and a temperature profile of200/230/240/220/220/220/220/220/220/220° C. with the β-nucleatedpropylene copolymer of 2.1.

Inside of the crosshead a steel pipe (Ø150 mm), coated with a 25 μmepoxy resin layer and a 30 μm compatibilizing layer of a maleic acidanhydride grafted propylene polymer (0.20% by weight of maleic acidanhydride), being preheated to a temperature of 190° C., is drivenforward with a speed of 1.2 m/min. The crosshead is designed so that thefoamed melt is added first onto the coated steel pipe, followed by themelt of the unfoamed propylene polymer for the outer layer, just beforethe pipe enters the vacuum calibration sleeve, which is cooled by waterof +20° C.

From the polyolefin coated steel pipe test specimens of a length of 254mm are machine cut. The polyolefin foam layer has a thickness of 55 mmand a density of 700 kg/m³. The unfoamed cover layer has a thickness of8 mm. The compressive strength of coated steel pipe test specimens (ASTMD 695-96, 5% compression) being 17 MPa.

Example 3

3.1 Preparation of the β-Nucleated Propylene Copolymer

A mixture of 75 wt % of a propylene block copolymer obtained by combinedbulk and gas phase polymerization using a Ziegler-Natta catalyst systemwith dicyclopentyldimethoxysilane as external donor, having an ethylenecontent of 8.3 wt %, an IRτ of the propylene homopolymer block of 0.985,and a melt index of 0.30 g/10 min at 230° C./2.16 kg, 25 wt % of amaster batch comprising 99.5 parts by weight of a propylene blockcopolymer having an ethylene content of 8.3 wt %, an IRτ of thepropylene homopolymer block of 0.985, and a melt index of 0.30 g/10 minat 230° C./2.16 kg, and 0.5 parts by weight of hexahydrophthaloylglycinecalcium salt, and 0.1 wt % calcium stearate, 0.1 wt %tetrakismethylene(3,5-di-t-butylhydroxyhydrocinnamate)]methane and 0.1wt % tris-(2,4-di-t-butylphenyl)phosphite, based on the sum of thepropylene copolymers used, is melted in a twin screw extruder with atemperature profile of 100/145/185/210/220/225/225/200/185° C.,homogenized, discharged and pelletized.

The resulting propylene copolymer has a melt index of 0.32 g/10 min at230° C./2.16 kg, a tensile modulus of 1310 MPa and a Charpy impactstrength using notched test specimens at −20° C. of 37 kJ/m².

3.2 Manufacture of the Propylene Copolymer Test Pipe

For producing the propylene copolymer test pipe, the β-nucleatedpropylene polymer of 3.1 is introduced in a single screw extruder(L/D=30, D=70 mm, temperature profile 200/210/220/220/220/220/200° C.,40 rpm), melted, extruded through a ring shaped die with a diameter of110 mm, taken off over a vacuum calibrating sleeve as a pipe of adiameter of 110 mm and a wall thickness of 10 mm, and cooled in a 6 mwater bath at +20° C., the taking off velocity being 0.3 m/min.

Rapid crack propagation test shows a critical pressure of 31 bar and adynamic fracture toughness of 19.60 MNm^(−3/2).

3.3 Manufacture of the Polyolefin Coated Steel Pipe

The pilot steel pipe coating line consists of a preheating unit,crosshead with extruder, vacuum calibration sleeve, cooling unit andcutting unit.

The crosshead is fed by a single screw extruder with a screw diameter of60 mm, an L/D of 35 and a temperature profile of200/230/240/220/220/220/220/220/220/220° C. with the β-nucleatedpropylene copolymer of 3.1.

Inside of the crosshead a steel pipe (Ø150 mm), coated with a 25 μmepoxy resin layer and a 30 μm compatibilizing layer of a maleic acidanhydride grafted propylene polymer (0,20 wt % maleic acid anhydride),being preheated to a temperature of 190° C., is driven forward with aspeed of 1.2 m/min. The crosshead is designed so that the melt of theβ-nucleated propylene copolymer is added onto the coated steel pipe,just before the pipe enters the vacuum calibration sleeve, which iscooled by water of +20° C.

The polyolefin coating has a thickness of 7.5 mm.

1. Polyolefin coated steel pipes having high dynamic fracture toughnessof the coating of the steel pipes during installation handling and inservice, comprising a steel pipe core and a polyolefin coating, thepolyolefin coating consisting essentially of β-nucleated propylenecopolymers of from 90.0 to 99.9 wt % of propylene and 0.1 to 10.0 wt %of α-olefins of 2 or 4 to 18 carbon atoms and having melt indices of 0.1to 8 g/10 min at 230° C./2.16 kg, a test polyolefin pipe fabricated fromthe β-nucleated propylene copolymer having a critical pressure of >25bars and a dynamic fracture toughness of >3.5 MNm^(−3/2).
 2. Polyolefincoated steel pipes according to claim 1, wherein the β-nucleatedpropylene copolymers comprise β-nucleated propylene block copolymershaving an IRτ of the propylene homopolymer block of ≧0.98, a tensilemodulus of ≧1100 MPa and a Charpy impact strength, using notched testspecimens at −20° C., of ≧6 kJ/m².
 3. Polyolefin coated steel pipesaccording to claim 2, wherein the β-nucleated propylene block copolymershaving an IRτ of the propylene homopolymer block of ≧0.98 comprisepropylene copolymers obtained by polymerization with a Ziegler-Nattacatalyst system comprising titanium-containing solid components, anorganoalumina, magnesium or titanium compound as cocatalyst and anexternal donor according to the formulaR_(x)R′_(y)Si(MeO)_(4-x-y), wherein R and R′ are identical or differentand are branched or cyclic aliphatic or aromatic hydrocarbon residues,and y and x independently from each other are 0 or 1, provided that x+yare 1 or
 2. 4. Polyolefin coated steel steel pipes according to claim 3,wherein the external donor comprises dicyclopentyldimethoxysilane. 5.Polyolefin coated steel steel pipes according to one of claims 1 to 4,wherein the β-nucleated propylene copolymers contain 0.0001 to 2.0 wt %,based on the propylene copolymers, dicarboxylic acid derivative typediamide compounds from C₅-C₈-cycloalkyl monoamines or C₆-C₁₂-aromaticmonoamines and C₅-C₈-aliphatic, C₅-C₈-cycloaliphatic or C₆-C₁₂-aromaticdicarboxylic acids, and/or diamine derivative diamide compounds fromC₅-C₈-cycloalkyl monocarboxylic acids or C₆-C₁₂-aromatic monocarboxylicacids and C₅-C₈-cycloaliphatic or C₆-C₁₂-aromatic diamines, and/or aminoacid derivative diamide compounds from amidation reaction ofC₅-C₈-alkyl-, C₅-C₈-cycloalkyl- or C₆-C₁₂-arylamino acids, C₅-C₈-alkyl-,C₅-C₈-cycloalkyl- or C₆-C₁₂-aromatic monocarboxylic acid chlorides andC₅-C₈-alkyl-, C₅-C₈-cycloalkyl- or C₆-C₁₂-aromatic monoamines, and/orquinacridone compounds, quinacridonequinone compounds, and/ordihydroquinacridone compounds, and/or dicarboxylic acid salts of metalsfrom group IIa of the periodic system and/or mixtures of dicarboxylicacids and metals from group IIa of the periodic system, and/or salts ofmetals from group IIa of periodic system and imido acids of the formula

wherein x=1 to 4; R═H, —COOH, C₁-C₁₂-alkyl, C₅-C₈-cycloalkyl orC₆-C₁₂-aryl, and Y═C₁-C₁₂-alkyl, C₅-C₈-cycloalkyl or C₆-C₁₂-aryl—substituted bivalent C₆-C₁₂-aromatic residues, as β-nucleating agent.6. Polyolefin coated steel pipes according to claim 10, wherein thefoamed plastic comprises foamed propylene copolymer having strainhardening behaviour and a melt index of 1.5 to 10 g/10 min at 230°C./2.16 kg.
 7. A process for producing polyolefin coated steel pipe ofclaim 1 or 10, comprising preheating the steel pipe and, while rotatingthe preheated steel pipe melt, applying each coating onto the rotatingpreheated steel pipe by means of respective coating extruders eachhaving a flat film die.
 8. A process for producing polyolefin coatedsteel pipe according to claim 7, wherein the extruder is a cone extruderthe temperature of a resultant melt of the copolymer at the ring die isfrom 195 to 240° C. and the temperature of the preheated steel pipe isfrom 160 to 200° C.
 9. Installation for off-shore transport of crude oilor gas products or district heating, comprising polyolefin coated steelpipe of claim 1 or
 10. 10. Polyolefin coated steel pipes of claim 1,further comprising an intermediate coating comprised of a foamedplastic.
 11. A process for producing polyolefin coated steel pipe ofclaim 1 or 10, comprising preheating the steel pipe and applying eachcoating onto the preheated steel pipe from an annular die of a crossheadfed by an extruder.